Multichannel interfacing device having a switching circuit

ABSTRACT

The invention relates to an interfacing device for transmission through interconnections used for sending a plurality of electrical signals. 
     The interfacing device of the invention comprises signal terminals and a common terminal. A transmitting circuit receives the input signals of the transmitting circuit coming from a source. The output of the transmitting circuit delivers, when the transmitting circuit is in the activated state, voltages between one of the signal terminals and the reference terminal (ground). A receiving circuit delivers, when the receiving circuit is in the activated state, output signals of the receiving circuit determined each by the voltage between one of the signal terminals and the common terminal, to the destination. In the closed state, the common terminal switching circuit is, for the common terminal, equivalent to a voltage source delivering a constant voltage, connected in series with a passive two-terminal circuit element presenting a low impedance.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a Continuation Application of PCT application PCT/IB2009/055287,filed 23 Nov. 2009, published in English under No. WO 2010/079390, whichin turn claims priority to French patent application No. 09/00042 filed8 Jan. 2009 and entitled “Dispositif d'interface multicanal avec circuitde commutation”, both of which are incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to an interfacing device for transmission throughinterconnections used for sending a plurality of electrical signals,such as the interconnections made with multiconductor cables, or withthe traces of a printed circuit board, or inside an integrated circuit.

PRIOR ART

Let us consider the problem of transmission through an interconnection,for obtaining m transmission channels, m being an integer greater thanor equal to 2. Each transmission channel may be used for transmittingsignals of any type, for instance analog signals or digital signals,from a source to a destination. We consider here that a digital signalis a signal whose value is defined only at discrete points in time, theset of the values that the signal may take on being discrete. Weconsider also that each value of a digital signal corresponds to avoltage or current interval. This definition of a digital signal as a“digital signal defined by voltage or current intervals” includes:

the binary signals used in binary signaling, that is to say any signalsuch that, in each transmission channel, the set of the values that thissignal may take on has 2 elements;the N-ary signals (N being an integer greater than or equal to 3) usedin multilevel signaling, that is to say any signal such that, in eachtransmission channel, the set of the values that this signal may take onhas N elements.

Binary signals are the signals which are the most frequently used todayby digital integrated circuits. Multilevel signals, for instancequaternary signals (sometimes referred to as PAM-4 or 4-PAM), are usedto obtain high bit rates. We will consider that any signal which doesnot comply with this definition of a digital signal is an analog signal.Thus, the result of any type of modulation of a carrier by a digitalsignal will be regarded as an analog signal.

Said transmission may be obtained through an interconnection comprisingm+1 conductors, among which m transmission conductors. We shall considerthree transmission impairments: echo, internal crosstalk and externalcrosstalk. Internal crosstalk refers to crosstalk within theinterconnection, between the different transmission channels. Externalcrosstalk refers to crosstalk involving couplings between theinterconnection and the external world.

The simplest transmission method for obtaining m transmission channelsuses m single-ended links. Using m single-ended links, each transmissionchannel uses one transmission conductor of the interconnection. In thismethod, echo may be reduced (but not canceled) using a suitabletermination. It is well known that this method is subject to internalcrosstalk.

The prior art concerning transmission without echo and without internalcrosstalk is set out in the 3 following patents:

the French patent number 0300064 of 6 Jan. 2003 entitled “Procédé etdispositif pour la transmission avec une faible diaphonie”,corresponding to the international application number PCT/EP2003/015036of 24 Dec. 2003 (WO 2004/062129), entitled “Method and device fortransmission with reduced crosstalk”;the French patent number 0302814 of 6 Mar. 2003 entitled “Procédé etdispositif numériques pour la transmission avec une faible diaphonie”,corresponding to the international application number PCT/EP2004/002382of 18 Feb. 2004 (WO 2004/079941), entitled “Digital method and devicefor transmission with reduced crosstalk”;the French patent number 0303087 of 13 Mar. 2003 entitled “Procédé etdispositif pour la transmission sans diaphonie”, corresponding to theinternational application number PCT/EP2004/002383 of 18 Feb. 2004 (WO2004/082168), entitled “Method and device for transmission withoutcrosstalk”.

As explained in the prior art section of the French patent applicationnumber 07/05260 of 20 Jul. 2007 entitled “Procédé et dispositif pour lestransmissions pseudo-différentielles”, corresponding to theinternational application number PCT/IB2008/052102 of 29 May 2008 (WO2009/013644), entitled “Method and device for pseudo-differentialtransmission”, a transmission scheme using the reference conductor,which is often called the ground conductor, as a return path for thereturn current produced by the currents flowing in the transmissionconductors, often suffers from a strong coupling between saidtransmission channels and some loops including a path in the referenceconductor. This particular case of external crosstalk is sometimescalled “ground noise” or “ground bounce”.

If the interconnection is used for building m single-ended links, thereference conductor (ground) is used for the return current produced bythe currents flowing on the m transmission conductors. This scheme isconsequently vulnerable to external crosstalk (it is also subject tointernal crosstalk).

If the interconnection is used according to one of the inventionsdescribed in said French patents number 0300064, number 0302814 andnumber 0303087 and the corresponding international applications, thereference conductor (ground) is also used for the return currentproduced by the currents flowing on the m transmission conductors. Theseinventions, which are suitable for reducing or eliminating internalcrosstalk, are therefore prone to external crosstalk.

However, there are transmission methods intended to provide a goodprotection against external crosstalk: differential links (see forinstance the book of H. W. Johnson and M. Graham entitled High-speeddigital design: a handbook of black magic, published by Prentice HallPTR in 1993), and pseudo-differential links (see for instance thesection II of the paper of A. Carusone, K. Farzan and D. A. Johnsentitled “Differential signaling with a reduced number of signal paths”published in IEEE Transactions on Circuits and Systems II, vol. 48,No._(—)3, pp. 294-300 in March 2001 and the section 4.2.3 of the book ofF. Yuan entitled CMOS current-mode circuits for data communications,published by Springer in 2007).

A differential device for transmission providing m transmission channelsuses an interconnection having n=2m transmission conductors. Apseudo-differential device for transmission providing m transmissionchannels uses an interconnection having n=m transmission conductors anda common conductor distinct from the reference conductor (ground). Thecommon conductor is referred to as “return conductor” in the case of thepseudo-differential transmission scheme disclosed in said French patentapplication number 07/05260 and the corresponding internationalapplication.

It should be noted that the wording “pseudo-differential” is alsoapplied to devices which are not related in any way topseudo-differential transmission. For instance, the patent applicationnumber US 2006/0267633 of the United States of America entitled“Pseudo-differential output driver with high immunity to noise andjitter” relates to a device having one differential input channel andone single-ended output channel: this device is not related topseudo-differential transmission in any way. For instance, the U.S. Pat.No. 5,638,322 of the United States of America entitled “Apparatus andmethod for improving common mode noise rejection in pseudo-differentialsense amplifiers” relates to sense amplifiers which to some extent looklike conventional differential amplifiers: this invention is not relatedto pseudo-differential transmission in any way.

The method disclosed in said French patent application number 07/05260and the corresponding international application is very effective forreducing external crosstalk and echo. However, this method does notreduce internal crosstalk. For instance, the article of F. Broydé and E.Clavelier entitled “A new pseudo-differential transmission scheme foron-chip and on-board interconnections” published in the proceedings ofthe “14^(éme) colloque international sur lacompatibilité´électromagnétique—CEM 08”, which took place in Paris,France, in May 2008, shows that this method does not provide a reductionof internal crosstalk. For instance, the article of F. Broydé and E.Clavelier entitled “Pseudo-differential links using a wide returnconductor and a floating termination circuit”, published in theproceedings of the “2008 IEEE International Midwest Symposium onCircuits and Systems (MWSCAS)”, which took place in Knoxville, Tenn., inthe United States of America, in August 2008, also indicates thatinternal crosstalk is present in a pseudo-differential link using themethod of said French patent application number 07/05260 and thecorresponding international application.

Conversely, as explained above, the inventions described in said Frenchpatents number 0300064, number 0302814 and number 0303087 and thecorresponding international applications are suitable for removinginternal crosstalk and echo, but they do not reduce external crosstalk.

The French patent application number 08/03985 of 11 Jul. 2008, entitled“Dispositif d'interface multicanal avec circuit d'équilibrage”,corresponding to the international application number PCT/IB2009/051557of 14 Apr. 2009, entitled “Multichannel interfacing device having abalancing circuit”, describes an interfacing device which may take theplace of a transmitting circuit used in one of the devices described insaid French patents number 0300064, number 0302814 and number 0303087,and the corresponding international applications, and may therebyprovide a reduction of external crosstalk. However, this interfacingdevice does not reduce the external crosstalk occurring in theinterconnection.

The French patent application number 08/04429 of 4 Aug. 2008, entitled“Procédé de transmission pseudo-différentiel utilisant des variablesélectriques modales”, corresponding to the international applicationnumber PCT/IB2009/052638 of 19 Jun. 2009, entitled “Method forpseudo-differential transmission using modal electrical variables”, andthe French patent application number 08/04430 of 4 Aug. 2008, entitled“Procédé de transmission pseudo-différentiel utilisant des variablesélectriques naturelles”, corresponding to the international applicationnumber PCT/IB2009/052645 of 19 Jun. 2009, entitled “Method forpseudo-differential transmission using natural electrical variables”,describe pseudo-differential devices for transmission which may be usedto reduce echo, internal crosstalk and external crosstalk. Such apseudo-differential device for transmission is shown in FIG. 1, thisdevice comprising an interconnection (1) having n=4 transmissionconductors and a return conductor (10) distinct from the referenceconductor (7). In FIG. 1, each end of the interconnection (1) isconnected to a termination circuit (4) which is not connected to thereference conductor (7). Three damping circuits (8) are connectedbetween the return conductor (10) and the reference conductor (7). Thetransmitting circuits (5) receive at their inputs the signals from them=4 channels of the two sources (2), and are connected to the conductorsof the interconnection (1). The receiving circuits (6) are connected tothe conductors of the interconnection (1). The pseudo-differentialdevice for transmission provides m transmission channels, such that thesignals of the m channels of a source (2) connected to a transmittingcircuit (5) in the activated state are sent to the m channels of thedestinations (3), without noticeable echo, internal crosstalk andexternal crosstalk. We note that the pseudo-differential device fortransmission shown in FIG. 1 provides bidirectional transmission.

Such a pseudo-differential device for transmission may comprise aninterfacing device described in said French patent application number08/03985 and the corresponding international application. The specialistsees that, in the case where bidirectional transmission is needed, thepseudo-differential device for transmission described in said Frenchpatent application number 08/04429 and the corresponding internationalapplication must, according to prior art, use an interfacing devicedescribed in said French patent application number 08/03985 and thecorresponding international application. For instance, the fifthembodiment of said French patent application number 08/04429 and thecorresponding international application, shown in FIG. 1, must use aninterfacing device described in said French patent application number08/03985 and the corresponding international application. The specialistalso sees that, in the same manner, this requirement applies to thepseudo-differential device for transmission described in said Frenchpatent application number 08/04430 and the corresponding internationalapplication, in the case where bidirectional transmission is needed andwhere the transmission variables delivered by a transmitting circuit arelinear combinations of signals each mainly determined by one and onlyone of the m “input signals of the transmitting circuit”, one or more ofthe linear combinations being such that two or more of the coefficientsof said one or more of the linear combinations are not equal to zero.

Unfortunately, it is not easy to build a fast and accurate balancingcircuit for the interfacing device described in said French patentapplication number 08/03985 and the corresponding internationalapplication. Additionally, the difficulty increases when smallertransistors are used, because of the low output impedance of MOSFETs indeep sub-micron CMOS technology. Consequently, it is often expensive tobuild high-performance interfacing devices for the pseudo-differentialdevices for transmission described in said French patent applicationsnumber 08/04429 and 08/04430, and the corresponding internationalapplications, in the case where bidirectional transmission is needed.

DESCRIPTION OF THE INVENTION

It is an object of the interfacing device of the invention to providetransmission through an interconnection having two or more transmissionconductors, the transmission presenting reduced external crosstalk andreduced internal crosstalk.

The invention is about a device for transmitting signals through aplurality of transmission channels, in a known frequency band,comprising:

m signal terminals, a common terminal and a reference terminal (ground),the signal terminals being intended to be connected to aninterconnection having at least m transmission conductors, m being aninteger greater than or equal to 2;a receiving circuit delivering, when the receiving circuit is in theactivated state, p “output signals of the receiving circuit”corresponding each to a transmission channel, p being an integer greaterthan or equal to 1 and less than or equal to m, the input of thereceiving circuit being coupled to at least p of said signal terminalsand to said common terminal, each of said “output signals of thereceiving circuit” being mainly determined by one or more of thevoltages between one of said signal terminals and said common terminal;a transmitting circuit receiving q “input signals of the transmittingcircuit” corresponding each to a transmission channel, q being aninteger greater than or equal to 2 and less than or equal to m, theoutput of the transmitting circuit being coupled to each of said signalterminals, the output of the transmitting circuit delivering, when thetransmitting circuit is in the activated state, m transmissionvariables, each of said transmission variables being either a voltagebetween one of said signal terminals and said reference terminal or acurrent flowing out of one of said signal terminals, each of saidtransmission variables being mainly determined by one or more of said“input signals of the transmitting circuit”, one or more of saidtransmission variables being not mainly determined by only one of said“input signals of the transmitting circuit”; anda common terminal switching circuit having an open state and a closedstate, the common terminal switching circuit having a return currentterminal connected to said common terminal, the common terminalswitching circuit being in the closed state when the transmittingcircuit is in the activated state, the common terminal switching circuitbeing in the open state when the receiving circuit is in the activatedstate, the common terminal switching circuit in the closed stateproviding, between said common terminal and said reference terminal, avoltage approximately equal to the sum of a constant voltage and of thevoltage, determined using the active sign convention, which would appearacross a first passive two-terminal circuit element through which thecurrent flowing from said return current terminal to said commonterminal would flow.

According to the invention, the common terminal is not connected to thereference terminal, in agreement with the principle ofpseudo-differential transmission. In the following, the wordings “is inthe deactivated state” and “is not in the activated state” areequivalent.

According to the invention, said “output signals of the receivingcircuit” may be analog signals or digital signals. According to theinvention, said “input signals of the transmitting circuit” may beanalog signals or digital signals.

According to the invention, said receiving circuit delivers “outputsignals of the receiving circuit” corresponding each to a transmissionchannel, when the receiving circuit is in the activated state. Accordingto the invention, there is a deactivated state of the receiving circuit,in which the common terminal switching circuit may be in the closedstate.

According to the invention, the transmitting circuit in the activatedstate delivers transmission variables, each of said transmissionvariables being mainly determined by one or more of said “input signalsof the transmitting circuit”, one or more of said transmission variablesbeing not mainly determined by only one of said “input signals of thetransmitting circuit”. According to the invention, there is adeactivated state of the transmitting circuit, in which the commonterminal switching circuit may be in the open state.

Consequently, according to the invention, said receiving circuit andsaid transmitting circuit cannot be simultaneously in the activatedstate, but said receiving circuit and said transmitting circuit may besimultaneously in the deactivated state. When the receiving circuit isin the activated state, the output of the transmitting circuit in thedeactivated state must be such that the voltage between the commonterminal and ground may vary (this is necessary for apseudo-differential link) and such that the voltages between the signalterminals coupled to the input of the receiving circuit and said commonterminal may vary. However, there is no similar general requirementrelating to the output of the receiving circuit in the deactivatedstate, according to the definition given above of a device of theinvention.

According to the invention, each of said transmission variables ismainly determined by one or more of said “input signals of thetransmitting circuit”, one or more of said transmission variables beingnot mainly determined by only one of said “input signals of thetransmitting circuit”. This must be interpreted in a broad sense, as:each of said transmission variable is mainly determined, at each pointin time, by the history, up to said point in time, of one or more ofsaid “input signals of the transmitting circuit”, one or more of saidtransmission variables being not mainly determined by the history, up tosaid point in time, of only one of said “input signals of thetransmitting circuit”.

Consequently, a linear combination of elements x₁, . . . , x_(r) being asum λ₁x₁+ . . . +λ_(r)x_(r) where λ₁, . . . , λ_(r) are the coefficientsof the linear combination, the specialist understands that, according tothe invention, each of said transmission variables delivered by saidtransmitting circuit may be a linear combination of signals each mainlydetermined by one and only one of said “input signals of thetransmitting circuit”, at least one of said linear combinations beingsuch that two or more of the coefficients of said at least one of saidlinear combinations are not equal to zero.

The transmitting circuit used in a device of the invention may forinstance only use analog signal processing to produce said transmissionvariables. It is also possible that the transmitting circuit used in adevice of the invention uses digital signal processing to produce saidtransmission variables.

The specialist also understands that, according to the invention, eachtransmission variable delivered by said transmitting circuit may bemainly determined by a linear combination of filtered “input signals ofthe transmitting circuit”, one or more of said transmission variablesbeing not mainly determined by one and only one of said filtered “inputsignals of the transmitting circuit”, each of said filtered “inputsignals of the transmitting circuit” being the result of the applicationof linear filtering to one of said “input signals of the transmittingcircuit”. Each of said filtered “input signals of the transmittingcircuit” may be substantially equal to the corresponding input signal ofthe transmitting circuit. Consequently, according to the invention, eachof said transmission variables delivered by said transmitting circuitmay be mainly determined by a linear combination of said “input signalsof the transmitting circuit”, at least one of said linear combinationsbeing such that two or more of the coefficients of said at least one ofsaid linear combinations are not equal to zero.

For said receiving circuit, each of said “output signals of thereceiving circuit” is mainly determined by one or more of the voltagesbetween one of said signal terminals and said common terminal. This mustbe interpreted in a broad sense, as: each of said “output signals of thereceiving circuit” is mainly determined, at each point in time, by thehistory, up to said point in time, of one or more of the voltagesbetween one of said signal terminals and said common terminal.Consequently, the specialist understands that, according to theinvention, each of said “output signals of the receiving circuit” may bemainly determined by a linear combination of “filtered voltages betweenone of said signal terminals and said common terminal”, each of said“filtered voltages between one of said signal terminals and said commonterminal” being the result of the application of linear filtering to oneof the voltages between one of said signal terminals and said commonterminal.

Each of said “filtered voltages between one of said signal terminals andsaid common terminal” may be substantially equal to the correspondingvoltage between one of said signal terminals and said common terminal.Consequently, according to the invention, each of said “output signalsof the receiving circuit” may be mainly determined by a linearcombination of the voltages between one of said signal terminals andsaid common terminal.

According to the invention, the wording “voltage, determined using theactive sign convention, which would appear across a two-terminal circuitelement through which the current flowing from said return currentterminal to said common terminal would flow” obviously refers to themost general “voltage, determined using the active sign convention,which may appear across a two-terminal circuit element subject to thecurrent flowing from said return current terminal to said commonterminal”, that is to say a voltage which, at each point in time, issolely determined by the history, up to said point in time, of thecurrent flowing from said return current terminal to said commonterminal. The specialist understands that this wording does not implythat a two-terminal circuit element subject to the current flowing fromsaid return current terminal to said common terminal is actually presentin a device of the invention: the two-terminal circuit element is onlyused for modeling the voltage between said common terminal and saidreference terminal (this voltage being equal to the voltage between saidreturn current terminal and said reference terminal).

According to the invention, said common terminal switching circuit inthe closed state behaves approximately, for said common terminal, as afirst passive two-terminal circuit element having a first terminal heldat a fixed voltage with respect to said reference terminal, and having asecond terminal connected to said return current terminal. Said fixedvoltage may be positive, negative or zero.

A device of the invention may be such that, in the open state, thecommon terminal switching circuit provides a current flowing out of saidreturn current terminal approximately equal to zero. However, this isnot at all a characteristic of the invention.

A device of the invention may be such that, in the open state, saidcommon terminal switching circuit provides a current flowing from saidreturn current terminal to said common terminal approximately equal tothe sum of a constant current and of the current which would flow out ofa second passive two-terminal circuit element subject to the voltagebetween said common terminal and said reference terminal, the product ofthe absolute value of the small-signal impedance (also referred to asdynamic impedance) of said first passive two-terminal circuit element ata first quiescent operating point, by the absolute value of thesmall-signal admittance (also referred to as dynamic admittance) of saidsecond passive two-terminal circuit element at a second quiescentoperating point being, in at least a part of said known frequency band,less than or equal to ½.

In this case, said common terminal switching circuit in the open statebehaves approximately, for said common terminal, as a second passivetwo-terminal circuit element connected in parallel with a current sourcedelivering a fixed current, this second passive two-terminal circuitelement having a first terminal connected to said reference terminal,and having a second terminal connected to said return current terminal.Said fixed current may be positive, negative or zero.

The wording “current which would flow out of a two-terminal circuitelement subject to the voltage between said common terminal and saidreference terminal” used above obviously refers to the most general“current flowing out of a two-terminal circuit element subject to thevoltage between said common terminal and said reference terminal”, thatis to say a current which, at each point in time, is solely determinedby the history, up to this point in time, of the voltage between saidcommon terminal and said reference terminal (this voltage being equal tothe voltage between said return current terminal and said referenceterminal). The specialist understands that this wording does not implythat a two-terminal circuit element subject to the voltage between saidcommon terminal and said reference terminal is actually present in adevice of the invention: the two-terminal circuit element is only usedfor modeling the current flowing out of said common terminal.

According to the invention, said first two-terminal circuit element usedfor modeling the common terminal switching circuit in the closed stateand said second two-terminal circuit element used for modeling thecommon terminal switching circuit in the open state are passivetwo-terminal circuit elements in the meaning of circuit theory (apassive circuit element is a circuit element in which the energyabsorbed can only be positive or zero), but these passive two-terminalcircuit elements are not necessarily linear.

According to the invention, said common terminal switching circuit inthe closed state may be such that, at said first quiescent operatingpoint, said first passive two-terminal circuit element has asmall-signal impedance having, in said known frequency band, an absolutevalue less than or equal to three hundred ohms. The set of the dynamicimpedances defined by this inequality is such that, when saidtransmitting circuit is in the activated state, the currents injected inthe transmission conductors may be associated with return currentsflowing mainly in the common conductor of said interconnection, whichmust be connected to said common terminal. The specialist understandsthat this situation allows reduced unwanted couplings with otherelectronic circuits near the interconnection.

The above inequality concerning the product of the absolute value of thesmall-signal impedance of said first passive two-terminal circuitelement by the absolute value of the small-signal admittance of saidsecond passive two-terminal circuit element should be applicable to thenormal operation of the device of the invention. Consequently:

said first quiescent operating point chosen for determining thesmall-signal impedance of said first passive two-terminal circuitelement should correspond to a quiescent current, flowing out of saidreturn current terminal, which may appear at a given point in time undernormal operation, when the common terminal switching circuit is in theclosed state;said second quiescent operating point chosen for determining thesmall-signal admittance of said second passive two-terminal circuitelement should correspond to a quiescent voltage, between said commonterminal and said reference terminal, which may appear at a given pointin time under normal operation, when the common terminal switchingcircuit is in the open state.

A device of the invention may also be such that, in the open state, saidcommon terminal switching circuit provides a current flowing out of saidreturn current terminal approximating the sum of a constant current andof the current which would flow out of a second passive two-terminalcircuit element subject to the voltage between said common terminal andsaid reference terminal, the product of the absolute value of thesmall-signal impedance of said first passive two-terminal circuitelement at any quiescent operating point taken in a specified intervalof currents flowing out of said return current terminal, by the absolutevalue of the small-signal admittance of said second passive two-terminalcircuit element at any quiescent operating point taken in a specifiedinterval of voltages between said common terminal and said referenceterminal being, in at least a part of said known frequency band, lessthan or equal to ½.

Let us use v_(C0 ON) to denote said constant voltage and let us usei_(C0 OFF) to denote said constant current. According to the invention,v_(C0 ON) may be positive, zero or negative and i_(C0 OFF) may bepositive, zero or negative. Let us use [i_(C1), i_(C2)] to denote saidspecified interval of currents flowing out of said return currentterminal and let us use [v_(C1), v_(C2)] to denote said specifiedinterval of voltages between said common terminal and said referenceterminal, in the case where these intervals are closed. According to theinvention, we may say that, for any frequency f in said part of saidknown frequency band:

1) for any quiescent operating point i_(C BIAS)∈[i_(C1), i_(C2)] of thecommon terminal switching circuit in the closed state, we may define asmall-signal impedance of said first passive two-terminal circuitelement, this small-signal impedance Z_(ON)(i_(C BIAS),f) being ingeneral a complex number;2) for any quiescent operating point v_(C BIAS)∈[v_(C1), v_(C2)] of thecommon terminal switching circuit in the open state, we may define asmall-signal admittance of said second passive two-terminal circuitelement, this small-signal admittance Y_(OFF)(v_(C BIAS), f) being ingeneral a complex number;3) we may have

  (1)

In other words, we have

either

  (2)

or

  (3)

In a way, we may say that, for the common terminal, in said part of saidknown frequency band, the absolute value of the dynamic impedance of thecommon terminal switching circuit in the closed state is less than orequal to the half of the absolute value of the dynamic impedance of thecommon terminal switching circuit in the open state.

According to the invention, when the common terminal switching circuitis in the closed state, the relationship between the current flowing outof said return current terminal and the voltage between said commonterminal and said reference terminal may be non-linear. Conversely,according to the invention, when the common terminal switching circuitis in the closed state, the relationship between the current flowing outof said return current terminal and the voltage between said commonterminal and said reference terminal may be linear. In this case, wemay, at a given frequency f, write

if f=0   (4)

if f≠0   (5)

where Z_(ON) is the internal impedance of the common terminal switchingcircuit in the closed state, where the voltage v _(C) is the phasorcorresponding to the real voltage v_(C) which is the voltage betweensaid common terminal and said reference terminal, and where the currenti _(C) is the phasor corresponding to the real current i_(C) which isthe current flowing out of said return current terminal.

According to the invention, when said common terminal switching circuitis in the open state, the relationship between the current flowing outof said return current terminal and the voltage between said commonterminal and said reference terminal may be non-linear. Conversely,according to the invention, when said common terminal switching circuitis in the open state, the relationship between the current flowing outof said return current terminal and the voltage between said commonterminal and said reference terminal may be linear. In this case, wemay, at a given frequency f, write

if f=0   (6)

if f≠0   (7)

where Y_(OFF) is the internal admittance of the common terminalswitching circuit in the open state.

We note that the equations (4) to (7) are not restricted to smallsignals and that they do not refer to a quiescent operating point.

In the case where the equation (1) and the equations (4) to (7) areapplicable, for any frequency f in said part of said known frequencyband, we note that the equation (1) may be replaced by:

  (8)

In this case, we may say that, in a way, for the common terminal:

the common terminal switching circuit in the closed state is equivalentto a network comprising a voltage source delivering a constant voltageconnected in series with a first passive linear two-terminal circuitelement presenting a “low” impedance;the common terminal switching circuit in the open state is equivalent toa network comprising a current source delivering a constant currentconnected in parallel with a second passive linear two-terminal circuitelement presenting a “high” impedance.

The specialists understand how they can build a common terminalswitching circuit used in the interfacing device of the invention.

A device of the invention may be such that said common terminalswitching circuit, said transmitting circuit and said receiving circuitare without any part in common to any two of them. Consequently, adevice of the invention may be such that said common terminal switchingcircuit has no part in common with said transmitting circuit.Conversely, an interfacing device of the invention may be such that saidcommon terminal switching circuit, said transmitting circuit and saidreceiving circuit are not without any part in common to any two of them.In particular, a device of the invention may be such that said commonterminal switching circuit has at least one part in common with saidtransmitting circuit.

Even in the case of a device of the invention in which said commonterminal switching circuit, said transmitting circuit and said receivingcircuit are not without any part in common to any two of them, thespecialist understands that the functions of the common terminalswitching circuit, of the transmitting circuit and of the receivingcircuit are distinct. The definition of a device of the invention, thisdefinition being based on the presence of a common terminal switchingcircuit, of a transmitting circuit and of a receiving circuit, musttherefore be understood as a definition relating to functions.

Said interconnection having m transmission conductors may be realizedusing a cable. Said interconnection may also be realized without using acable, for instance an interconnection formed in or on a rigid orflexible printed circuit board (using traces and/or copper areas), or aninterconnection formed in or on the substrate of a multi-chip module(MCM) or of an hybrid circuit, or an interconnection formed inside amonolithic integrated circuit.

A device of the invention may be such that it constitutes a part of anintegrated circuit, said interconnection being realized inside saidintegrated circuit. In this case, it is possible that said m signalterminals and/or said common terminal are not coupled to pins of saidintegrated circuit.

A device of the invention may be such that it constitutes a part of anintegrated circuit, each of said m signal terminals being coupled to oneor more pins of said integrated circuit, said common terminal beingcoupled to one or more pins of said integrated circuit. Thisconfiguration is appropriate when said interconnection is realizedoutside said integrated circuit. The specialist notes that if there aremany signal terminals, for instance more than 16 signal terminals, theabsolute value of the current which may flow through the common terminalmay become much larger than the maximum absolute value of the currentflowing through a single signal terminal. Consequently, in this case, ifa single pin is allocated to the common terminal, a degradation oftransmission may occur for fast signals, because of the inductance of aconnection using a single pin. In this case, using several pins for thecommon terminal reduces this inductance and improves transmission.

A device of the invention may further comprise a termination circuitcoupled to each of said signal terminals and to said common terminal,the termination circuit being, when the termination circuit is in theactivated state, approximately equivalent, for said signal terminals andsaid common terminal, to a (m+1)-terminal network such that, at at leastone quiescent operating point, for small signals in said part of saidknown frequency band, the impedance matrix, with respect to said commonterminal, of said (m+1)-terminal network is equal to a wanted diagonalmatrix of size m×m. Consequently, if each of said “output signals of thereceiving circuit” is mainly determined by one and only one of thevoltages between one of said signal terminals and said common terminal,a device of the invention may comprise a “Pseudo-differentialinterfacing device having a termination circuit” described in the Frenchpatent application number 07/04421 of 21 Jun. 2007, entitled “Dispositifd'interface pseudo-différentiel avec circuit de terminaison”,corresponding to the international application number PCT/IB2008/051826of 8 May 2008 (WO 2008/155676), entitled “Pseudo-differentialinterfacing device having a termination circuit”.

A device of the invention may further comprise a termination circuitcoupled to each of said signal terminals and to said common terminal,the termination circuit being, when the termination circuit is in theactivated state, approximately equivalent, for said signal terminals andsaid common terminal, to a (m+1)-terminal network such that, at at leastone quiescent operating point, for small signals in said part of saidknown frequency band, the impedance matrix, with respect to said commonterminal, of said (m+1)-terminal network is equal to a wantednon-diagonal matrix of size m×m. Consequently, a device of the inventionmay comprise a “Multichannel interfacing device having a terminationcircuit” disclosed in the French patent application number 08/03876 of 8Jul. 2008, entitled “Dispositif d'interface multicanal avec circuit determination”, corresponding to the international application numberPCT/IB2009/051182 of 20 Mar. 2009, entitled “Multichannel interfacingdevice having a termination circuit”.

According to the invention, it is possible that there is a deactivatedstate of the termination circuit, in which the behavior of thetermination circuit is different from the one defined above. However,the existence of a deactivated state of the termination circuit is notat all a characteristic of the invention.

An interfacing device of the invention may be such that said terminationcircuit is made of a network of resistors.

A termination circuit made of a network of resistors is however not atall a characteristic of the invention. By way of a first example,designers may, in order to reduce the power consumed by the terminationcircuit, choose to allow the termination circuit to be effective only ina relevant interval of frequencies, for instance by including suitablereactive circuit elements in the termination circuit. By way of a secondexample, the termination circuit could include active components, forinstance insulated gate field-effect transistors (MOSFETs) operating inthe ohmic regime. The impedance of the channel of such components may beadjustable by electrical means. Consequently, said termination circuitmay be such that the impedance matrix, with respect to said commonterminal, of said termination circuit in the activated state can beadjusted by electrical means.

In the same way, a device of the invention may be such that saidsmall-signal impedance of said first passive two-terminal circuitelement and/or said small-signal admittance of said second passivetwo-terminal circuit element can be adjusted by electrical means.

In the case where the termination circuit has an activated state and adeactivated state, the impedance of the channel of one or more MOSFETsmay for instance be controlled by one or more control signals taking ondifferent values in the activated state and in the deactivated state.Consequently, said termination circuit may be such that said terminationcircuit has an activated state and a deactivated state, the impedancematrix, with respect to said common terminal, of said terminationcircuit in the activated state being different from the impedancematrix, with respect to said common terminal, of said terminationcircuit in the deactivated state.

In the case where the termination circuit has an activated state and adeactivated state, components such as transistors may for instance beused as switches having a closed state and an open state. In this case,said transistors may for instance be in the closed state when thetermination circuit is in the activated state, and be in the open statewhen the termination circuit is in the deactivated state. Consequently,said termination circuit may be such that said termination circuit hasan activated state and a deactivated state, each current flowing fromsaid termination circuit to one of said signal terminals beingsubstantially zero when said termination circuit is in the deactivatedstate. Designers may, in order to reduce the power consumed by thetermination circuit, choose to put such a termination circuit in thedeactivated state when said transmitting circuit is in the activatedstate.

An interfacing device of the invention may be such that said terminationcircuit has no part in common with said common terminal switchingcircuit and/or with said transmitting circuit and/or with said receivingcircuit. Conversely, an interfacing device of the invention may be suchthat said termination circuit has one or more parts in common with saidcommon terminal switching circuit and/or with said transmitting circuitand/or with said receiving circuit.

According to the invention, the number m of signal terminals may beequal to the number q of “input signals of the transmitting circuit”.According to the invention, the number m of signal terminals may beequal to the number p of “output signals of the receiving circuit”. Inparticular, an interfacing device of the invention may be such that m isgreater than or equal to three.

According to the invention, the q “input signals of the transmittingcircuit” may for instance be applied to the transmitting circuit usingsingle-ended links. According to the invention, the q “input signals ofthe transmitting circuit” may for instance be applied to thetransmitting circuit using differential links. According to theinvention, the p “output signals of the receiving circuit” may forinstance be delivered using single-ended links. According to theinvention, the p “output signals of the receiving circuit” may forinstance be delivered using differential links.

According to the invention, the transmitting circuit and/or thereceiving circuit may have a filtering function, for instance for thepurpose of obtaining a pre-emphasis, a de-emphasis or an equalizationimproving transmission. It then becomes necessary to synthesize thecorresponding filters, either as analog filters or as digital filters,using one of the many methods known to specialists.

When losses are not negligible in the interconnection, phase andamplitude distortions may occur, which are referred to as distortionscaused by propagation. The reduction of these distortions may beobtained, in a device of the invention, using an equalization reducingthe effects of the distortions caused by propagation, said equalizationbeing implemented in said transmitting circuit and/or in said receivingcircuit. This type of processing, which is also sometimes referred to ascompensation, is well known to specialists, and may be implemented usinganalog signal processing or digital signal processing. Specialists knowthat it is commonplace to use adaptive algorithms for implementing thistype of processing in receivers for data transmission. A device of theinvention may use an adaptive equalization. This type of processing iswell known to specialists, and is often implemented using digital signalprocessing.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and characteristics of the invention will appear moreclearly from the following description of particular embodiments of theinvention, given by way of non-limiting examples, with reference to theaccompanying drawings in which:

FIG. 1 shows a pseudo-differential system for transmission comprising aninterconnection having four transmission conductors, and has alreadybeen discussed in the section dedicated to the presentation of priorart;

FIG. 2 shows the block-diagram of a first embodiment of the invention;

FIG. 3 shows a use of a device of the invention;

FIG. 4 shows the block-diagram of a second embodiment, a thirdembodiment and a fourth embodiment;

FIG. 5 shows a use of a device of the invention;

FIG. 6 shows the transmitting circuit used in the third embodiment.

DETAILED DESCRIPTION OF SOME EMBODIMENTS First Embodiment

As a first embodiment of an interfacing device of the invention, givenby way of non-limiting example, we have represented in FIG. 2 aninterfacing device of the invention built inside an integrated circuit,comprising m=4 signal terminals (101) and a common terminal (100), thesignal terminals (101) and the common terminal (100) being intended tobe connected to an interconnection having m transmission conductors.

A transmitting circuit (5) receives q=4 “input signals of thetransmitting circuit” coming from a source (2), the output of thetransmitting circuit being coupled to the m signal terminals (101). Theoutput of the transmitting circuit (5) is not coupled to the commonterminal (100). The output of the transmitting circuit (5) delivers,when the transmitting circuit is in the activated state, m transmissionvariables, each transmission variable being a voltage between one ofsaid signal terminals (101) and the reference terminal (ground), eachtransmission variable being a linear combination of signals each mainlydetermined by one and only one of said q “input signals of thetransmitting circuit”, at least one of said linear combinations beingsuch that two or more of the coefficients of said at least one of saidlinear combinations are not equal to zero. When the transmitting circuit(5) is in the activated state, its output presents a low impedancebetween said signal terminals (101) and said reference terminal. Whenthe transmitting circuit (5) is not in the activated state, its outputpresents a high impedance, so that the transmitting circuit (5) does notproduce transmission variables and only causes a negligible currentthrough the signal terminals (101).

A receiving circuit (6) delivers, when the receiving circuit is in theactivated state, p=4 “output signals of the receiving circuit”corresponding each to a transmission channel, the input of the receivingcircuit being coupled to the m signal terminals (101) and to the commonterminal (100), each of said “output signals of the receiving circuit”being mainly determined by a linear combination of the voltages betweenone of said signal terminals (101) and said common terminal (100). Theinput of the receiving circuit (6) always presents a high impedancebetween said signal terminals (101) and said reference terminal. The“output signals of the receiving circuit” are delivered to thedestination (3) when the receiving circuit (6) is in the activatedstate. When the receiving circuit (6) is not in the activated state, itsoutput presents a high impedance, so that the receiving circuit (6) doesnot deliver any “output signal of the receiving circuit”.

The specialist knows several suitable methods for producing a highimpedance state at the output of the transmitting circuit (5) and at theoutput of the receiving circuit (6). The specialist understands how hemay design, with prior art techniques, a transmitting circuit (5)performing said linear combinations of signals each mainly determined byone and only one of said “input signals of the transmitting circuit”, atleast one of said linear combinations being such that two or more of thecoefficients of said at least one of said linear combinations are notequal to zero, using analog signal processing and/or digital signalprocessing to perform said linear combinations. For instance, suchtransmitting circuits are used in said French patent number 0300064,said French patent number 0302814 and the corresponding internationalapplications. The specialist understands how he may design, with priorart techniques, a receiving circuit (6) such that each of said “outputsignals of the receiving circuit” is mainly determined by a linearcombination of the voltages between one of said signal terminals (101)and said common terminal (100), using analog signal processing and/ordigital signal processing to perform said linear combinations. Forinstance, such receiving circuits are disclosed in the French patentapplication number 08/03830 of 7 Jul. 2008, entitled “Circuit deréception pseudo-différentiel” corresponding to the internationalapplication number PCT/IB2009/051053 of 13 Mar. 2009, entitled“Pseudo-differential receiving circuit”.

In the device of the invention shown in FIG. 2, the receiving circuit(6) and the transmitting circuit (5) cannot be simultaneously in theactivated state. The possibility of controlling the activated state of atransmitting circuit and/or of a receiving circuit is usually used indata bus architectures. We note that the circuits needed to control theactivated state of the transmitting circuit (5) and of the receivingcircuit (6) at a given point in time are not shown in FIG. 2. We alsonote that the address and/or control lines necessary for coordinatingthe activated state of the transmitting circuit (5) and of the receivingcircuit (6) with the operation of the other entities connected to such abus are not shown in FIG. 2. These address and/or control lines could beconductors of said interconnection.

A common terminal switching circuit (9) has a return current terminalconnected to said common terminal (100). The common terminal switchingcircuit (9) has an open state and a closed state. The common terminalswitching circuit (9) is in the closed state when the transmittingcircuit (5) is in the activated state, and is in the open state when thereceiving circuit (6) is in the activated state. When the transmittingcircuit (5) is in the deactivated state and the receiving circuit (6) isin the deactivated state, the common terminal switching circuit (9) isfor instance in the open state.

In the closed state, the common terminal switching circuit (9) providesa voltage v_(C) between said common terminal (100) and said referenceterminal (ground), this voltage v_(C) approximating the sum of aconstant voltage v_(C0 ON) and of the voltage, determined using theactive sign convention, which would appear across a first passivetwo-terminal circuit element subject to the current flowing from saidreturn current terminal to said common terminal. In other words, in theclosed state, the common terminal switching circuit (9) is, for thecommon terminal (100), equivalent to a network comprising a voltagesource delivering a constant voltage v_(C0 ON), this voltage sourcehaving a first terminal connected to ground, this voltage source havinga second terminal connected to the first terminal of the first passivetwo-terminal circuit element, the second terminal of the first passivetwo-terminal circuit element being connected to said common terminal(100).

In the open state, the common terminal switching circuit (9) provides acurrent flowing out of said return current terminal approximating thesum of a constant current i_(C0 OFF) and of the current which would flowout of a second passive two-terminal circuit element subject to thevoltage between said common terminal (100) and said reference terminal.In other words, in the open state, the common terminal switching circuit(9) is, for the common terminal (100), equivalent to a networkcomprising a current source delivering a constant current i_(C0 OFF),this current source having a first terminal connected to ground, thiscurrent source having a second terminal connected to said commonterminal (100), this current source being connected in parallel with thesecond passive two-terminal circuit element.

At any frequency f such that f<1 GHz, the equation (1) is applicable,that is to say: the product of |Z_(ON)(i_(C BIAS),f)| by|Y_(OFF)(v_(C BIAS),f)| is less than or equal to ½.

Let us consider a use of a device of the invention A, this use alsoinvolving a device of the invention B identical to the device of theinvention A for receiving the transmission variables sent by the deviceof the invention A through said interconnection. We therefore considerthe case in which the transmitting circuit of the device of theinvention A is in the activated state and the receiving circuit of thedevice of the invention B is in the activated state. The FIG. 3 shows anequivalent network (81) for the circuit element having m+2 terminalsseen by the left end of the interconnection (83) when the transmittingcircuit of the device of the invention A is in the activated state. Thisequivalent network (81) for the device of the invention A having itstransmitting circuit in the activated state comprises a first isolatedcircuit (811) having exactly m+1 terminals, a voltage source (813)delivering the constant voltage v_(C0 ON) and a first passivetwo-terminal circuit element (812) presenting a dynamic impedanceZ_(ON)(i_(C BIAS),f). The first isolated circuit (811) having exactlym+1 terminals has m terminals connected to the signal terminals (1011)of the device of the invention A, has one terminal connected to groundand presents a low impedance between each of the signal terminals (1011)of the device of the invention A and ground. The common terminal (1001)of the device of the invention A is connected to the first passivetwo-terminal circuit element (812) connected in series with the voltagesource (813).

The FIG. 3 also shows an equivalent network (82) for the circuit elementhaving m+2 terminals seen by the right end of the interconnection (83)when the receiving circuit of the device of the invention B is in theactivated state. This equivalent network (82) for the device of theinvention B having its receiving circuit in the activated statecomprises a second isolated circuit (821) having exactly m+1 terminals,a current source (823) delivering the constant current i_(C0 OFF) and asecond passive two-terminal circuit element (822) presenting a dynamicadmittance Y_(OFF)(v_(C BIAS),f). The second isolated circuit (821)having exactly m+1 terminals has m terminals connected to the signalterminals (1012) of the device of the invention B and has one terminalconnected to the common terminal (1002) of the device of the inventionB. The common terminal (1002) of the device of the invention B is alsoconnected to the second passive two-terminal circuit element (822)connected in parallel with the current source (823).

According to a first example, in a given frequency band,|Z_(ON)(i_(C BIAS),f)|<10Ω and |Y_(OFF)(v_(C BIAS),f)|<100 μS.Consequently, |Z_(ON)(i_(C BIAS),f)||Y_(OFF)(v_(C BIAS),f)|< 1/1000. Inthis first example, the common terminal (1001) of the device of theinvention A presents a “low” impedance with respect to ground and thecommon terminal (1002) of the device of the invention B presents a“high” impedance with respect to ground.

According to a second example, in a given frequency band,Z_(ON)(i_(C BIAS),f)≈78Ω and |Y_(OFF)(v_(C BIAS),f)|<120 μS.Consequently, |Z_(ON)(i_(C BIAS),f)| |Y_(OFF)(v_(C BIAS),f)|< 1/100. Inthis second example, the common terminal (1002) of the device of theinvention B presents a “high” impedance with respect to ground and thefirst passive two-terminal circuit element (812) effectively damps theresonances of the common conductor of the interconnection (83), whichlinks the common terminals (1001) (1002). Here, Z_(ON)(i_(C BIAS), f) issuch that the receiving circuit of the device of the invention B musteffectively reject the influence of the voltage between its commonterminal (1002) and ground on the “output signals of the receivingcircuit”.

According to a third example, in a given frequency band,|Z_(ON)(i_(C BIAS), f)|<5Ω and Y_(OFF)(v_(C BIAS),f)≈6 mS. Consequently,|Z_(ON)(i_(C BIAS),f)| |Y_(OFF)(v_(C BIAS),f)|< 3/100. In this thirdexample, the common terminal (1001) of the device of the invention Apresents a “low” impedance with respect to ground and the second passivetwo-terminal circuit element (822) effectively damps the resonances ofthe common conductor of the interconnection (83).

The specialists may compare the advantages of these three examples ofproportioning of the first passive two-terminal circuit element (812)and of the second passive two-terminal circuit element (822), as afunction of the length and of the characteristics of the interconnection(83), and as a function of the considered frequency band.

The specialist understands that, for an accurate modeling of a givenuse, it would be necessary to describe propagation and couplings in theinterconnection (83), for instance using the theory of multiconductortransmission lines to take into account the distributed capacitances andthe distributed inductances. The equivalent networks (81) (82) of FIG. 3for the devices of the invention are also simplified. The equivalentschematic diagram of FIG. 3 is therefore only an approximation.

The specialist understands how he may, using prior art techniques,design a common terminal switching circuit (9) providing an open statewhen the receiving circuit (6) is in the activated state and a closedstate when the transmitting circuit (5) is in the activated state. Forinstance, such common terminal switching circuits are used in the Frenchpatent application number 07/04949 of 9 Jul. 2007, entitled “Dispositifd'interface pseudo-différentiel avec circuit de commutation”,corresponding to the international application PCT/IB2008/051982 of 20May 2008 (WO 2009/007866) entitled “Pseudo-differential interfacingdevice having a switching circuit”.

Second Embodiment

As a second embodiment of an interfacing device of the invention, givenby way of non-limiting example, we have represented in FIG. 4 aninterfacing device of the invention built inside an integrated circuit,comprising m=4 signal terminals (101) and a common terminal (100), thesignal terminals (101) and the common terminal (100) being intended tobe connected to an interconnection having m transmission conductors.

A transmitting circuit (5) receives q=4 “input signals of thetransmitting circuit” coming from a source (2), the output of thetransmitting circuit being coupled to the m signal terminals (101). Theoutput of the transmitting circuit (5) is not coupled to the commonterminal (100). The output of the transmitting circuit (5) delivers,when the transmitting circuit is in the activated state, m transmissionvariables, each transmission variable being a voltage between one ofsaid signal terminals (101) and the reference terminal (ground), eachtransmission variable being mainly determined by a linear combination ofsaid q “input signals of the transmitting circuit”, at least one of saidlinear combinations being such that two or more of the coefficients ofsaid at least one of said linear combinations are not equal to zero.

A receiving circuit (6) delivers, when the receiving circuit is in theactivated state, p=4 “output signals of the receiving circuit”corresponding each to a transmission channel, the input of the receivingcircuit being coupled to the m signal terminals (101) and to the commonterminal (100). When the receiving circuit (6) is in the activatedstate, the “output signals of the receiving circuit” are delivered tothe destination (3).

A common terminal switching circuit (9) has a return current terminalconnected to said common terminal (100).

This second embodiment of an interfacing device of the inventioncomprises a termination circuit (4), the termination circuit beingconnected to the m signal terminals (101) and to the common terminal(100), the termination circuit being, when the termination circuit is inthe activated state, approximately equivalent, for said signal terminalsand said common terminal, to a linear (m+1)-terminal network such that,in the known frequency band used for transmission, the impedance matrix,with respect to said common terminal, of said linear (m+1)-terminalnetwork is equal to a wanted diagonal matrix of size m×m. This wanteddiagonal matrix of size m×m may be frequency-dependent.

In this second embodiment, each of said “output signals of the receivingcircuit” is mainly determined by one and only one of the voltagesbetween one of said signal terminals (101) and said common terminal(100). Consequently, the termination circuit (4) and the receivingcircuit (6) form a “Pseudo-differential interfacing device having atermination circuit” disclosed in said French patent application number07/04421 and the corresponding international application.

FIG. 5 shows a use of interfacing devices of the invention, this usecomprising an interconnection (1) having m=4 transmission conductors(11) (12) (13) (14) and a return conductor (10), the interconnectionbeing structurally combined with a reference conductor (7), that is tosay a ground conductor. All items shown in FIG. 5 belong to the sameprinted circuit assembly and the reference conductor (7) is a groundplane of the printed circuit board of this printed circuit assembly.Said transmission conductors (11) (12) (13) (14) and the returnconductor (10) are traces built in the printed circuit board. At eachend of the interconnection (1), we find an interfacing device of thesecond embodiment of the invention, comprising a transmitting circuit(5), a common terminal switching circuit (9), a receiving circuit (6)and a termination circuit (4). The FIG. 5 also shows a source (2) and adestination (3) at each end of the interconnection (1).

When the transmitting circuit (5) of the interfacing device of theinvention shown on the left in FIG. 5 is in the activated state, thereceiving circuit (6) of the interfacing device of the invention shownon the right in FIG. 5 must be in the activated state. In this case, thecurrent flowing in the reference conductor (7) because of the signalssent through the interconnection (1) is low since only one commonterminal switching circuit (9) is in the closed state (the commonterminal switching circuit shown on the left in FIG. 5). When thetransmitting circuit (5) of the interfacing device of the inventionshown on the right in FIG. 5 is in the activated state, the receivingcircuit (6) of the interfacing device of the invention shown on the leftin FIG. 5 must be in the activated state. In this case, the currentflowing in the reference conductor (7) because of the signals sentthrough the interconnection (1) is low since only one common terminalswitching circuit (9) is in the closed state (the common terminalswitching circuit shown on the right in FIG. 5). Consequently, thespecialist in electromagnetic compatibility understands that the objectof reduced external crosstalk is achieved.

The interconnection (1) and the two interfacing devices of the secondembodiment of the invention shown in FIG. 5 are proportioned such thatthey form a device for pseudo-differential transmission disclosed insaid French patent application number 07/05260 and the correspondinginternational application. Consequently, a reduced echo is obtained.However, the impedance of the return conductor (10) and the impedance ofa common terminal switching circuit (9) in the closed state produce someinternal crosstalk (this phenomenon is similar to the one explained inthe first embodiment of said French patent application number 08/03830and the corresponding international application). The specialistunderstands that the linear combinations of signals each mainlydetermined by one and only one of said q “input signals of thetransmitting circuit”, performed in one of the transmitting circuits (5)when this transmitting circuit is in the activated state, can be used toreduce this cause of internal crosstalk.

Third Embodiment (Best Mode)

A third embodiment of an interfacing device of the invention, given byway of non-limiting example and best mode of carrying out the invention,also corresponds to the interfacing device of the invention shown inFIG. 4, comprising m=4 signal terminals (101) and a common terminal(100), the signal terminals (101) and the common terminal (100) beingintended to be connected to an interconnection having m transmissionconductors.

A transmitting circuit (5) receives q=4 “input signals of thetransmitting circuit” coming from a source (2), the output of thetransmitting circuit being coupled to the m signal terminals (101). Theoutput of the transmitting circuit (5) is not coupled to the commonterminal (100). The output of the transmitting circuit (5) delivers,when the transmitting circuit is in the activated state, m transmissionvariables, each transmission variable being a current flowing out of oneof said signal terminals (101), each transmission variable being alinear combination of signals each mainly determined by one and only oneof said q “input signals of the transmitting circuit”, at least one ofsaid linear combinations being such that two or more of the coefficientsof said at least one of said linear combinations are not equal to zero.When the transmitting circuit (5) is in the activated state, its outputpresents a high impedance between said signal terminals (101) and saidreference terminal. When the transmitting circuit (5) is not in theactivated state, its output also presents a high impedance and onlycauses a negligible current through the signal terminals (101).

A receiving circuit (6) delivers, when the receiving circuit is in theactivated state, p=4 “output signals of the receiving circuit”corresponding each to a transmission channel, the input of the receivingcircuit being coupled to the m signal terminals (101) and to the commonterminal (100). The “output signals of the receiving circuit” aredelivered to the destination (3).

A common terminal switching circuit (9) has a return current terminalconnected to said common terminal (100).

This third embodiment of an interfacing device of the inventioncomprises a termination circuit (4), the termination circuit beingconnected to the m signal terminals (101) and to the common terminal(100), the termination circuit being, when the termination circuit is inthe activated state, approximately equivalent, for said signal terminalsand said common terminal, to a linear (m+1)-terminal network such that,in the known frequency band used for transmission, the impedance matrix,with respect to said common terminal, of said (m+1)-terminal network isequal to a wanted non-diagonal matrix of size m×m. This wantednon-diagonal matrix of size m×m may be frequency-dependent. Thetermination circuit (4) and the receiving circuit (6) form a“Multichannel interfacing device having a termination circuit” disclosedin said French patent application 08/03876 and the correspondinginternational application.

In this third embodiment, each of said “output signals of the receivingcircuit” is mainly determined by a linear combination of the voltagesbetween one of said signal terminals and said common terminal, at leastone of said linear combinations being such that two or more of thecoefficients of said at least one of said linear combinations are notequal to zero.

The transmitting circuit (5) is shown in FIG. 6, in which:

each of the q “input signals of the transmitting circuit” is applied toan input (57) which is a differential input comprising two terminals(571) (572);each of the q inputs (57) is connected to an input of a multiple-inputand multiple-output signal processing device (521) having q inputs and moutputs;each of the m outputs of said multiple-input and multiple-output signalprocessing device (521) corresponds to an input of a voltage-to-currentconverter (transconductor) comprising a resistor (511) connected to theinput of a current mirror made of two n-channel transistors (512) (513);the output of each of said current mirrors is biased by a current source(514) supplying a practically constant current;each of the m signal terminals (101) is connected to the drain of theoutput transistor (513) of each of said current mirrors.

The specialist understands that the current sources (514) shown in FIG.6 are ideal circuit elements which may be realized with real components,for instance using current mirrors.

Each output voltage of the multiple-input and multiple-output signalprocessing device (521) is a linear combination of signals each mainlydetermined by one and only one of said q “input signals of thetransmitting circuit”, at least one of said linear combinations beingsuch that two or more of the coefficients of said at least one of saidlinear combinations are not equal to zero.

For instance, said multiple-input and multiple-output signal processingdevice (521) may be a multiple-input and multiple-output amplifiersimilar to one of the transmitting circuits described in said Frenchpatent number 0300064 and the corresponding international application,for instance one of the transmitting circuits shown in FIG. 7 or FIG. 9of said French patent number 0300064 and the corresponding internationalapplication.

For instance, said multiple-input and multiple-output signal processingdevice (521) may be a digital signal processing device similar to one ofthe transmitting circuits described in said French patent number 0302814and the corresponding international application, for instance thetransmitting circuit shown in FIG. 2 of said French patent number0302814 and the corresponding international application in the casewhere the “input signals of the transmitting circuit” are digitalsignals, or the transmitting circuit shown in FIG. 4 of said Frenchpatent number 0302814 and the corresponding international application inthe case where the “input signals of the transmitting circuit” areanalog signals.

We may now consider another use of interfacing devices of the invention,shown in FIG. 5, such that, at each end of the interconnection (1), wefind an interfacing device of the third embodiment of the invention,comprising a transmitting circuit (5), a common terminal switchingcircuit (9), a receiving circuit (6) and a termination circuit (4).

The interconnection (1) and the two interfacing devices of the thirdembodiment of the invention shown in FIG. 5 are proportioned such thatthey form a device for pseudo-differential transmission using modalelectrical variables disclosed in said French patent application number08/04429 and the corresponding international application. Consequently,a reduced echo and a reduced internal crosstalk are obtained.

Fourth Embodiment

A fourth embodiment of an interfacing device of the invention, given byway of non-limiting example, is intended for sending and receivingbinary digital signals. This fourth embodiment also corresponds to theinterfacing device of the invention shown in FIG. 4, comprising m=4signal terminals (101) and a common terminal (100), the signal terminals(101) and the common terminal (100) being intended to be connected to aninterconnection having m transmission conductors.

A transmitting circuit (5) receives q=4 “input signals of thetransmitting circuit” coming from a source (2), the output of thetransmitting circuit being coupled to the m signal terminals (101). Theoutput of the transmitting circuit (5) is not coupled to the commonterminal (100). The output of the transmitting circuit (5) delivers,when the transmitting circuit is in the activated state, m transmissionvariables, each transmission variable being a current flowing out of oneof said signal terminals (101), each transmission variable being mainlydetermined by a linear combination of said q “input signals of thetransmitting circuit”, at least one of said linear combinations beingsuch that two or more of the coefficients of said at least one of saidlinear combinations are not equal to zero. The transmitting circuit (5)is a multiple-input and multiple-output amplifier disclosed in theFrench patent application no. 06/00388 of 17 Jan. 2006 entitled“Amplificateur à entrées multiples et sorties multiples”, correspondingto the international application no. PCT/IB2006/003950 of 19 Dec. 2006(WO 2007/083191), entitled “multiple-input and multiple-outputamplifier” or in the French patent application no. 08/03982 of 11 Jul.2008 entitled “Amplificateur à entrées multiples et sorties multiplesayant des entrées pseudo-différentielles”, corresponding to theinternational application no. PCT/IB2009/051358 of 31 Mar. 2009,entitled “multiple-input and multiple-output amplifier havingpseudo-differential inputs”.

A receiving circuit (6) delivers, when the receiving circuit is in theactivated state, p=4 “output signals of the receiving circuit”corresponding each to a transmission channel, the input of the receivingcircuit being coupled to the m signal terminals (101) and to the commonterminal (100). The “output signals of the receiving circuit” aredelivered to the destination (3). Each of said “output signals of thereceiving circuit” is mainly determined by one and only one of thevoltages between one of said signal terminals (101) and said commonterminal (100). The receiving circuit (6) may be of any suitable knowntype of pseudo-differential receiver for digital signals, for instanceone of the pseudo-differential receivers described in the patent of theUnited States of America U.S. Pat. No. 5,994,925 entitled“Pseudo-differential logic receiver” or in the patent of the UnitedStates of America U.S. Pat. No. 7,099,395 entitled “Reducing couplednoise in pseudo-differential signaling”.

A common terminal switching circuit (9) has a return current terminalconnected to said common terminal (100).

This fourth embodiment of an interfacing device of the inventioncomprises a termination circuit (4), the termination circuit beingconnected to the m signal terminals (101) and to the common terminal(100), the termination circuit being, when the termination circuit is inthe activated state, approximately equivalent, for said signal terminalsand said common terminal, to a linear (m+1)-terminal network such that,in the known frequency band used for transmission, the impedance matrix,with respect to said common terminal, of said (m+1)-terminal network isequal to a wanted non-diagonal matrix of size m×m. The terminationcircuit (4) and the receiving circuit (6) form a “Multichannelinterfacing device having a termination circuit” disclosed in saidFrench patent application 08/03876 and the corresponding internationalapplication.

We may now consider another use of interfacing devices of the invention,shown in FIG. 5, such that, at each end of the interconnection (1), wefind an interfacing device of the fourth embodiment of the invention,comprising a transmitting circuit (5), a common terminal switchingcircuit (9), a receiving circuit (6) and a termination circuit (4).

The interconnection (1) and the two interfacing devices of the fourthembodiment of the invention shown in FIG. 5 are proportioned such thatthey form a device for pseudo-differential transmission using naturalelectrical variables disclosed in said French patent application number08/04430 and the corresponding international application. Consequently,a reduced echo and a reduced internal crosstalk are obtained.

INDICATIONS ON INDUSTRIAL APPLICATIONS

The interfacing device of the invention is suitable forpseudo-differential transmission between integrated circuits through aninterconnection having two or more transmission conductors, thetransmission presenting reduced unwanted couplings.

We note that, in the embodiment of an interfacing device of theinvention, given above by way of non-limiting example and shown in FIG.6, the active components are MOSFETs. This is not at all acharacteristic of the invention, and specialists understand that itwould have also been possible to use bipolar transistors or other typesof active components. Consequently, the interfacing device of theinvention may be implemented in integrated circuits made using anyapplicable manufacturing process.

The invention is suitable for the protection against the noise producedby unwanted electromagnetic couplings in printed circuit boards. Theinvention is particularly advantageous to printed circuit boardscomprising wide-band analog circuits or fast digital circuits. Forsending in q transmission channels, the invention has the advantage ofonly requiring q+1 pins on an integrated circuit providing the functionsof the transmitting circuit, of the receiving circuit and of the commonterminal switching circuit, as opposed to 2q pins in the case of atransceiver for differential transmission.

The interfacing device of the invention is particularly suitable forpseudo-differential transmission inside an integrated circuit, becauseit provides a good protection against the noise related to the currentsflowing in the reference conductor and in the substrate of theintegrated circuit.

An interfacing device of the invention may be built inside an integratedcircuit, but this is not at all a characteristic of the invention. Forinstance, it could be interesting that the transmitting circuit, thereceiving circuit and the common terminal switching circuit be builtinside an integrated circuit, a termination circuit being built outsidethis integrated circuit.

Since the invention includes the possibility of a deactivated state forthe terminals intended to be connected to said interconnection, forinstance a high impedance state, the invention is suitable for animplementation in a data bus architecture.

The invention is particularly suitable for multilevel signaling, becausethis type of transmission scheme is more sensitive to noise than binarysignaling.

1. A device for transmitting signals through a plurality of transmissionchannels, in a known frequency band, comprising: m signal terminals, acommon terminal and a reference terminal, the signal terminals beingintended to be connected to an interconnection having at least mtransmission conductors, m being an integer greater than or equal to 2;a receiving circuit delivering, when the receiving circuit is in theactivated state, p output signals of the receiving circuit correspondingeach to a transmission channel, p being an integer greater than or equalto 1 and less than or equal to m, the input of the receiving circuitbeing coupled to at least p of the signal terminals and to the commonterminal, each of the output signals of the receiving circuit beingmainly determined by one or more of the voltages between one of thesignal terminals and the common terminal; a transmitting circuitreceiving q input signals of the transmitting circuit corresponding eachto a transmission channel, q being an integer greater than or equal to 2and less than or equal to m, the output of the transmitting circuitbeing coupled to each of the signal terminals, the output of thetransmitting circuit delivering, when the transmitting circuit is in theactivated state, m transmission variables, each of the transmissionvariables being either a voltage between one of the signal terminals andthe reference terminal or a current flowing out of one of the signalterminals, each of the transmission variables being mainly determined byone or more of the input signals of the transmitting circuit, one ormore of the transmission variables being not mainly determined by onlyone of the input signals of the transmitting circuit; and a commonterminal switching circuit having an open state and a closed state, thecommon terminal switching circuit having a return current terminalconnected to the common terminal, the common terminal switching circuitbeing in the closed state when the transmitting circuit is in theactivated state, the common terminal switching circuit being in the openstate when the receiving circuit is in the activated state, the commonterminal switching circuit in the closed state providing, between thecommon terminal and the reference terminal, a voltage approximatelyequal to the sum of a constant voltage and of the voltage, determinedusing the active sign convention, which would appear across a firstpassive two-terminal circuit element subject to the current flowing fromthe return current terminal to the common terminal.
 2. The device fortransmitting signals of claim 1, wherein each of the transmissionvariables delivered by the transmitting circuit is a linear combinationof signals each mainly determined by one and only one of the inputsignals of the transmitting circuit, at least one of the linearcombinations being such that two or more of the coefficients of said atleast one of the linear combinations are not equal to zero.
 3. Thedevice for transmitting signals of claim 1, wherein each of thetransmission variables delivered by the transmitting circuit is mainlydetermined by a linear combination of the input signals of thetransmitting circuit, at least one of the linear combinations being suchthat two or more of the coefficients of said at least one of the linearcombinations are not equal to zero.
 4. The device for transmittingsignals of claim 1, wherein, in the open state, the common terminalswitching circuit provides a current flowing from the return currentterminal to the common terminal approximately equal to the sum of aconstant current and of the current which would flow out of a secondpassive two-terminal circuit element subject to the voltage between thecommon terminal and the reference terminal, the product of the absolutevalue of the small-signal impedance of said first passive two-terminalcircuit element at a first quiescent operating point, by the absolutevalue of the small-signal admittance of said second passive two-terminalcircuit element at a second quiescent operating point being, in a partof the known frequency band, less than or equal to ½.
 5. The device fortransmitting signals of claim 4, wherein the common terminal switchingcircuit in the closed state is such that, at the first quiescentoperating point, the first passive two-terminal circuit element has asmall-signal impedance having, in the known frequency band, an absolutevalue less than or equal to three hundred ohms.
 6. The device fortransmitting signals of claim 1, wherein the small-signal impedance ofthe first passive two-terminal circuit element can be adjusted byelectrical means.
 7. The device for transmitting signals of claim 4,wherein the small-signal impedance of the second passive two-terminalcircuit element can be adjusted by electrical means.
 8. The device fortransmitting signals of claim 1, wherein the device for transmittingsignals constitutes a part of an integrated circuit, the interconnectionbeing realized inside the integrated circuit.
 9. The device fortransmitting signals of claim 1, wherein the device for transmittingsignals constitutes a part of an integrated circuit, each of the signalterminals being coupled to one or more pins of the integrated circuit,the common terminal being coupled to one or more pins of the integratedcircuit.
 10. The device for transmitting signals of claim 1, furthercomprising a termination circuit coupled to each of the signal terminalsand to the common terminal, the termination circuit being, when thetermination circuit is in the activated state, approximately equivalent,for the signal terminals and the common terminal, to a (m+1)-terminalnetwork such that, at at least one quiescent operating point, for smallsignals in a part of the known frequency band, the impedance matrix,with respect to the common terminal, of the (m+1)-terminal network isequal to a wanted matrix of size m×m.
 11. The device for transmittingsignals of claim 10, wherein the impedance matrix, with respect to thecommon terminal, of the termination circuit in the activated state canbe adjusted by electrical means.
 12. The device for transmitting signalsof claim 10, wherein the termination circuit has an activated state anda deactivated state, each current flowing from the termination circuitto one of the signal terminals being substantially zero when thetermination circuit is in the deactivated state.